docs: def vtt_talk_sources(ttFont) -> List[str]:
    """Return the tags of VTT source tables found in a font."""
    VTT_SOURCE_TABLES = {'TSI0', 'TSI1', 'TSI2', 'TSI3', 'TSI5'}
    tables_found = [tag for tag in ttFont.keys() if tag in VTT_SOURCE_TABLES]
    return tables_found